In one carburetor of this kind, an idling fuel jet, an acceleration jet, a ventilation jet, and a main jet are provided one behind the other in the venturi. At the idle setting for the throttle valve, the idle fuel outlet jet lies in front of the throttle valve in the direction of the air flow, that is, on the side facing toward the motor; whereas, the acceleration jet and the main jet lie behind the throttle valve on the opposite side.
During idle, fuel is drawn in by suction only through the idling jet. When, during acceleration, the throttle valve is pivoted, it sweeps over the acceleration jet after a slight rotating movement, so that this jet too lies in front of the throttle valve. The fuel then flows from the idling jet as well as from the acceleration jet. When the throttle valve is opened further, and finally lies in line with the flow direction, the greatest part of the fuel is drawn by suction from the main jet.
The idle adjusting part serves to fix the position of the throttle valve in that it fixes the throttle valve in the idling position. This idle adjusting part is an abutment screw on which the throttle valve lies with a pivot arm. To change the idling speed, the throttle valve is pivoted by means of this abutment screw whereby the available air gap between the throttle valve and the inner wall surface of the venturi is made smaller or larger. This has the disadvantage that the acceleration jet lying behind the throttle valve is disposed at only a small spacing from the throttle flap. In this way, the throttle valve enters the area of the acceleration jet with only minimal pivoting thereby causing fuel to flow from this jet which can lead to a considerable change in the mixture. In order to compensate for this, an idling fuel screw must be adjusted thereafter in order to diminish the quantity of fuel moving through the idling fuel jet. Consequently, both adjustment screws must be set with respect to each other in order to attain the optimum setting for the idling mixture, which is troublesome and time consuming, and requires a certain amount of experience on behalf of the operator.
Also, when the acceleration jet lies at a greater spacing behind the throttle valve, this complicated adjustment of both adjustment screws is unavoidable. In this situation, the throttle valve is easily adjusted, since a slight pivoting movement does not cause the valve to reach the area of the acceleration jet; however, the mixture will still be altered by a slight change in the position of the throttle valve.
Opening or closing the throttle valve will enlarge or diminish the air gap thereby causing the amount of entering air to vary correspondingly. The fuel-air mixture therefore becomes too lean or too rich by a slight adjustment of the throttle valve, which must again be compensated for by a follow-up adjustment of the fuel adjustment screw. Consequently, in both cases, the idling adjustment screw as well as the idling fuel screw must be adjusted with respect to each other to obtain the optimum idling adjustment which is difficult and time consuming, especially for laymen.